Using nationally representative student assessment data from PISA 2018 for South Korea, this study analyzes the effects of between-student competition and cooperation at the school level on individual academic performance and adolescent well-being. Instrumental variables regressions were used to address potential endogeneity. Findings indicate that a more competitive school climate induces higher academic performance in mathematics, reading, and science. Student competition, however, comes at the cost of reduced individual life satisfaction. We find that competition is stronger at schools with a higher proportion of male and single children, and where mothers have higher education levels. Competition is found to increase the investment into after-school private tutoring. In contrast to competition, cooperation among students shows no significant effect on academic performance, while it has a positive effect on individual life satisfaction. Students at schools with higher levels of cooperation also show more positive affect, less negative affect, and perceive greater meaning in life.